This invention relates to a thermal transfer ink sheet. More particularly, this invention relates to a thermal transfer ink sheet which can prevent inferiority in superposition of colors (or simply "color shift") in color recording by a thermal transfer method.
Recently, thermal transfer recording for forming transferred images on plain paper by using a thermal printer, a thermal facsimile, or the like has been developed. This thermal transfer recording is noticed recently, because maintenance of the apparatus is easy due to simple mechanism, the price and maintenance cost of the apparatus are low, clear and fast recording can be obtained with a low energy, and color recording is relatively easy by using a multi-color ink sheet. In the color recording by using the thermal transfer method, desired multi-colored images are obtained by using hot-melt ink layers of yellow, magenta and cyan, and if necessary black, and melt transferring predetermined images of individual colors to image receiving papers in a predetermined order to superpose the colors. In such a case, colors are formed by single colors of yellow, magenta, cyan, and black, and red from yellow and magenta, green from yellow and cyan, blue from magenta and cyan, and black from yellow, magenta and cyan, depending on the size of heating body of a thermal head.
But there often takes place a color shift due to positional shift of printed images when two or more colors are printed over again in addition to defacement of images (a phenomenon of broadening images due to too low melting point and melt viscosity) and breaking of images (a phenomenon of narrowing images due to too high melting point and melt viscosity or badness in transfer) due to differences in physical properties such as a melting point, a melt viscosity, etc. of inks on ink sheets. In order to remove such a defect, there have been proposed various improvement on printing devices, ink sheets and image receiving sheets.
For example, when a red image is formed from yellow and magenta, the following phenomenon takes place. At the time of melt transfer of a magenta ink layer on a thermal transfer ink sheet, a yellow ink layer on a plain paper, which contacts with the magenta ink layer, also melts by the heat applied to the magenta ink layer and mix melted with the magenta ink layer and sticked so as to superpose the magenta ink on the yellow portion on the plain paper, followed by separation of the plain paper and the thermal transfer ink sheet. In such a case, the plain paper is raised up to the thermal transfer ink sheet side, and the position of the yellow on the plain paper and the transfer position of the magenta are shifted to cause the color shift. In order to solve the problem of color shift, Japanese patent Unexamined publication No. 58-158291 proposes to make melting temperatures of hot-melt ink layers of at least yellow, magenta and cyan formed on a substrate different stepwisely in accordance with the printing order by a thermal head.
But the color shift is not caused only by the difference in the melting points of two inks at the time of melting. Fundamentally, various physical properties between the two inks should be considered in order to solve the problem of color shift.
On the other hand, in order to solve problems of the color shift and inferiority in transfer at the time of printing, Japanese patent Unexamined publication No. 61-14973 proposes a transfer type thermal printer. According to this proposal, the thermal printer is constructed so as to send an ink sheet and a transfer sheet in contact with each other to a separating portion forwardly after the end of transfer at a transfer portion, to separate the two at the separating portion, and to send the transfer sheet backward to the original position after the separation of the two at the separating portion.
In order to conduct color printing without causing the color shift and to improve image quality with an apparatus in smaller in size and faster in printing speed, Japanese patent Unexamine publication No. 61-43871 proposes a color printer wherein a recording paper is fitted on a revolving body and thermal transfer recording is conducted by revolving the revolving body one time every time a printing color is changed.
But even if the printing apparatus are improved as mentioned above, the problem of color shift is not solved sufficiently. In other words, the phenomenon of color shift may be caused by some physical properties between the ink sheet and the image receiving paper and the improvement of the apparatus should be preceded by the improvement of such physical properties.